The present invention relates to liquid additive concentrates for thermoplastics, especially to liquid colorant concentrates which are compatable both with polyolefin resins and with a wide variety of other thermoplastic resins.
The traditional procedure for coloring plastic resins, or for introducing additives into a plastic, is to tumble blend the plastic resin with a solid colorant or additive concentrate, such as a coloring powder or polymer pellets containing a high concentration of a colorant or other additive. The blended material is then fed into the extruder or molding machine, where the resin and the additive are plastized and the colorant or additive is incorporated into the molten plastic.
While generally effective, this method had a number of drawbacks. In many cases, the colorant or additive concentrate does not uniformly mix with the plastic resin, or it does not disperse uniformly during molding. In some cases, it tends to segregate from the resin during pre-molding handling because of particle size or specific gravity differences. Any such non-uniformity of distribution tends to produce unacceptable finished products, either due to non-uniform color, such as color specks or streaks, or to unacceptable mechanical properties due to concentrations of colorant or additive in the finished product. In addition, the use of polymer pellet concentrates requires a molder to maintain a large inventory of colorant pellets for each different color and resin type used in his plant.
In recent years, liquid colorants have been developed in an effort to overcome the disadvantages of the traditional solid colorants. Being liquids, these colorants may be accurately metered directly into the molding machine or extruder by in-line metering systems, which help overcome many of the disadvantages of solid colorants.
Previous liquid colorants have been based on surfactants and wetting agents. E.g., U.S. Pat. Nos. 4,167,503 to Cipriani and 3,956,008 to Knepper, et al. The surfactant based vehicles in these colorants provide an excellent pigment dispersion media and allow relatively high pigment loadings, at viscosities suitable for use in standard in-line metering equipment.
These surfactant based colorant and additive systems normally work quite well in polar polymers. However, the surfactants used in these colorants are polar in nature and are incompatable with many plastics, most notably the polyolefins. This incompatability can cause poor mixing in the molder or extruder, resulting in non-uniform color and the presence of color defects, such as specks of colorant. These surfactant based liquid colorants also tend to produce "screw slippage" in the molding machine or extruder when used to color polyolefins and certain other plastics, especially at high colorant loadings. Moreover, incompatable vehicles have a tendency to exude, or spew, to the surface of the molded plastic.
Accordingly, there is a need for a non-surfactant based liquid colorant which is compatable with polyolefin resins while still possessing the other important characteristics of a good liquid colorant. For example, the vehicle for the colorant should be able to produce a stable dispersion with a wide variety of organic and inorganic pigments and dyes. It should be capable of retaining high pigment loadings, and should be stable at the temperatures normally encountered in plastics molding and extrusion equipment. The vehicle also should be compatable with a wide variety of commercial thermoplastic resins other than the polyolefins. The colorant should have a viscosity suitable for use in conventional in-line metering and blending equipment. It should not cause "screw slippage". It should produce uniform color products, without visible streaks or specks, and without impairing the mechanical properties of the finished product. The colorant also should be non-toxic and odor-free. Although principally intended for the preparation of liquid colorants, the vehicle should be capable of dispersing other standard additives for plastics, such as UV absorbers, antioxidants, blowing agents, and the like.